In Outlook go to Tools - Options - Mail Format. Under the 'Stationery and Fonts' heading, ensure it is set to 'None'.
It may also be worth checking that 'use Word to edit e-mail messages' at the top of this dialog is also ticked.
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Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.